Oracle has released the Linux base install repositories to the public. This means that, when you have obtained the ISOs for OEL4 or OEL5, you can install packages freely using yum. The so-called "base channels" are available for free and without support over Internet. This applies to Oracle VM (Xen) as well. All you need is their .repo files, yum check-update, and you're set.
Updates and Patches require a support license. Check Oracle Public Yum Server for details and howto.